Currently, portable terminals such as notebook personal computers, personal digital assistances (PDAs), and portable telephones are used daily at places where other people are around, e.g., on vehicles such as trains and airplanes, and various public facilities. It is expected that such trends will accelerate further in the future in accordance with increases in public LAN equipment caused due to developments of a ubiquitous information society. Under such environments, there are issues regarding security and privacy, i.e., displayed contents on the display can be recognized by people around. Further, there is an increasing tendency to enhance protection of confidential information such as establishing the Personal Information Protection Law, and it has been demanded to take measures such as protecting highly confidential information from those who are not entitled to view that information not only when using the portable terminal outside such as in the above cases but also when using desktop devices at workplaces or the like.
As one of the techniques for satisfying such demands, there is known a method which combines display of a plurality of images and an optical shutter that synchronizes with the display, for example (see patent Document 1). This technique will be described by referring to FIG. 26.
The method disclosed in Patent Document 1 uses the optical shutter that synchronizes with display of a plurality of images, with which only specified users can view prescribed images. This enables protection of the information. As shown in FIG. 26, the method switches and displays a secret image 271 that is the image originally desired to be displayed and an image 272 that is an inversed image of the secret image 271. At the same time, as shown in FIG. 26C, the optical shutter 273 used only by the user is turned into a transmission state 273a by synchronizing with the secret image 271, and the optical shutter 273 is turned into a light-shielding state 273b by synchronizing with the inversed image 272.
With this, the displayed image is recognized by the user of the optical shutter as a perceived image that is equivalent to the secret image 271 as in FIG. 26B, and it is recognized by surrounding unspecified people as a halftone (gray) perceived image 274 in which the inversed image 272 is superimposed on the secret image 271 as in FIG. 26A. Even if the halftone perceived image 274 is recognized, it is impossible for the unspecified people to identify the secret image 271 and only the specified person can recognize the secret image 271, since the whole perceived image 274 is gray.
However, with the method disclosed in Patent Document 1, the image to be kept confidential is displayed in an original form. Thus, there may be cases where even unspecified people that do not use the optical shutter can recognize the secret image, when the unspecified people blink their eyes at the moment where the secret image 271 is displayed or when the image is taken by a camera. As one of the techniques for overcoming this issue, there is proposed a secret image dispersive display method (see Patent Document 2).
As shown in FIG. 27, the method disclosed in Patent Document 2 for overcoming the issue pays attention to the fact that a letter “A” that is the secret image to be displayed in the first place (original image) is displayed by a set of a plurality of pixels 284. The method disperses an image into three images 286, 287, and 288 shown in FIG. 28 from which the pixels 284 configuring the letter “A” shown in FIG. 27 are thinned out, inserts a dummy image that is irrelevant to the secret image between the dispersed images 286, 287, and 288, and displays the images while switching those at a high speed.
In Patent Document 2, the three images 286, 287, and 288 from which the pixels 284 are thinned out as well as the dummy image are displayed while being switched at a high speed. At the same time, the timing for displaying those images and the timing for opening/closing the optical shutter are synchronized, and the optical shutter is set to a transmission state only when the dispersed secret images (divided images) are displayed. In this manner, Patent Document 2 utilizes an afterimage effect of human eyes so that only the permitted user can observe the secret images. Further, Patent Document 2 also discloses a technique which can transmit information only to a specific user with secret images, while displaying public images to unspecified people.
It is also considered to apply Patent Document 1 to Patent Document 2. FIG. 29 shows an example of such case. In the example shown in FIG. 29, a letter “A” that is the original image is dispersed into two images 291 and 293, and images 292 and 294 which are inversed images of the images 291 and 293 are inserted respectively to the images 291 and 293. Further, a letter “F” that is a public image 295 is inserted, and those images are displayed on a display while being switched at a high speed. As shown in FIG. 29A, unspecified people who do not use the shutter 296 recognize the letter “F” to which the images 291-295 are added as a perceived image 297. In the meantime, as shown in FIG. 29C, the specified person uses the shutter 296. Thus, by controlling the shutter 296 to be in the transmission state 296a when the divided images 291 and 293 are displayed and to be in the light-shielding state 296b when displaying the inversed images 292, 294 and the public image 295, the specified user can recognize the letter “A” to which the divided images 291 and 293 are added as the perceived image 298 as in FIG. 29B.
In the example shown in FIG. 29, even if the unspecified people perceive the divided image 291 or the divided image 294 by blinking or taking it by a camera, such image alone does not makes any sense. Thus, the confidentiality can be improved further. Patent Document 3 also proposes a method which divides original image data to N pieces (N≧2) of image data in order to prevent unlawful copy of the image data.
Patent Document 1: Japanese Unexamined Patent Publication 6-110403
Patent Document 2: Japanese Unexamined Patent Document 2001-255844
Patent Document 3: Japanese Patent No. 3577455
However, there are issues in the above-described methods disclosed in Patent Document 2 or Patent Document 3. Such issues will be described by referring to FIG. 30 which shows only the image displayed on the display taken out from FIG. 29.
In FIG. 30, it is assumed that the images are displayed in order of arrows, a series of two or more images (from a divided image 301 to a public image 305) are together called a frame, and repeated frequency is a frame frequency. Further, each image configuring the frame is called a field.
An image obtained by synthesizing the divided image 301 and the inversed image 302, and an image obtained by synthesizing the divided image 303 and the inversed image 304 are recognized as a halftone gray image, respectively. Further, in order for the image obtained by synthesizing the divided image 301 and the divided image 303 to be recognized as the perceived image 298 shown in FIG. 29, it is necessary to shorten the period from a writing start point of a certain image to a writing start point of a next image (i.e., a field cycle) when displaying images from the divided image 301 to the inversed image 304. Specifically, it is necessary to set the frame cycle to be short to be within a period where the afterimage effect of human eyes works or within a period where no flicker is generated.
In a liquid crystal display used in a PC (personal computer), normally, “1 frame=1 field” and the frame frequency is 60 Hz. If the number of fields is increased as in FIG. 30 while keeping the frame frequency as 60 Hz, the frequency per field becomes 300 Hz that is five times as that of the normal display provided that the display cycle of each field is the same. The driving frequency of the pixels also increases accordingly. Thus, when normal line sequential driving is executed on a display with resolution of XGA, the driving frequency of 200 MHz-300 MHz is required as the driving frequency of a data driver. For preventing the image from being mixed into a next image when viewing the images by using the optical shutter, if a part of the field cycle is used for writing and a backlight is lighted up at the point where the writing is ended, for example, the driving frequency becomes still higher.
FIG. 30 shows the example in which the secret image is dispersed into two. When the number of dispersed images is increased, naturally the frequency is increased further. Driving with such high frequency induces an increase of the power consumption. The driving frequency can be lowered by expanding the writing period. However, in that case, if the field cycles are fixed, the light-up period of the backlight becomes shortened. This results in causing an issue of deteriorating the brightness.
Even if a dot clock is simply increased for shortening the writing period further, the writing period cannot be shortened. Signal input time for the pixels is rate-determined with the data line wiring delay at the time of normal signal input, so that it is difficult to achieve driving with high frequency.
In the case of Patent Document 3, the driving frequency is also increased at least twice as that of the normal display due to dividing of the image or increased to several times more depending on the number of divisions. Therefore, Patent Document 3 has the same issues in terms of the fact that the power consumption is increased and the driving becomes difficult.
It is an object of the present invention to provide a display system and a display method, which can display images brightly by decreasing or without increasing the driving frequency of a controller and a driver which controls the display in a display system which switches and displays a plurality of images at a high speed.